study of refrigeration systems

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Study Of Refrigeration Systems & Their Comparative Study (2010-11)

By: Shetty Akhil U. (TE_MECH_P-47) Phakatkar Abhijit (TE_MECH_P-35) Krishnan Rhoheth (TE_MECH_P-41) Paunikar Shweta (TE_MECH_P-29)

Refrigeration & Air-Conditioning

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Schematic of a reverse Carnot refrigeration system using Air

Process 1-2: Reversible, adiabatic compression in a compressor (C),Process 2-3: Reversible, isothermal heat rejection in a compressor, Process 3-4: Reversible, adiabatic expansion in a turbine(T),Process 4-1: Reversible, isothermal heat absorption in a turbine.

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1. Air is refrigerant

2. Refrigerant remains in gaseous state throughout

3. Easiest way of cooling developed

4. Comparatively lower heat carrying capacity

5. Low equipment weight and volume so used in aircraft refrigeration systems

6. Basic element – compressor, cooler or heat exchanger, expander and refrigerator

7. Low coefficient of performance and high power requirement

• Introduction to Air Refrigeration System :

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• Limitations Of Air Refrigeration Cycle:

1. Employing a gas, is the difficulty, for achieving isothermal heat transfer during processes 2-3 and 4-1.

2. Low Speed: Isothermal Process High Speed: Isentropic Process, impossible

in a single cycle.

3. Cycle suffers from low COP as processes are irreversible.

4. Air used as medium, has low COP & Heat Absorption capacity.

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1. A vapour compression refrigeration system is an improved type of air refrigeration system in which a suitable working substance, termed as refrigerant, is used.

2. It condenses and evaporates at temperatures and pressures close to the atmospheric conditions.

3. The refrigerant used, does not leave the system, but is circulated throughout the system alternately condensing and evaporating.

4. In evaporating, the refrigerant absorbs its latent heat from the brine (salt water) which is used for circulating it around the cold chamber.

• Introduction To Simple VCRS :

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Schematic of a standard, saturated, single stage (SSS) vapour compression Refrigeration system

Process 1-2: Isentropic compression of saturated vapour in compressor Process 2-3: Isobaric heat rejection in condenser Process 3-4: Isenthalpic expansion of saturated liquid in expansion device Process 4-1: Isobaric heat extraction in the evaporator

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Standard Vapour Compression Refrigeration System(VCRS)

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• Advantages of VCRS:

1. Very mature technology.

2. Relatively inexpensive.

3. Can be driven directly using mechanical energy (water, car/truck motor) or with electrical energy.

4. Efficient up to 60% of Carnot's theoretical limit (as evaluated in ASHRAE testing conditions: evaporation temperature of -23.3 °C, condensing temperature of 54.4°C, and ambient temperature of 32°C)

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• Disadvantages of VCRS:

1. Still use HCFC refrigerants, which contribute to depletion of the Earth's ozone layer.

Countries adhering to Montreal Protocol, HCFCs are to be phased out by ozone-friendly HFCs, slightly less efficient than systems using HCFCs, CO2, butane.

Temperature–Entropy diagram of VCRS

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2. Initial cost high.

3. Prevention of leakage major problem.

4. Operation noisier due to moving parts, compared to VARS.

5. Mechanical energy used for changing the phase of refrigerant in evaporator.

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• Why Compound VCRS used?

As evaporator temperature decreases: i. Throttling losses increase ii. Superheat losses increase iii. Compressor discharge temperature increases iv. Quality of the vapour at the inlet to the evaporator increases v. Specific volume at the inlet to the compressor increases

Volumic refrigeration effect decreases rapidly as the specific volume increases, with decreasing evaporator temperature

Single stage systems not recommended when the evaporator temperature becomes very low and/or when the condenser temperature becomes high.

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1. The vapour refrigerant is required to be delivered at very high pressure as in case of low temperature refrigerating systems.

2. In such cases we need to compress the vapour refrigerant by employing the single stage compressor with a very high pressure ratio between the condenser & evaporator or compress it in two or more compressors placed in series.

3. The compression carried out in two or more compressors is called compound or multistage compression.

• Introduction

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4. In vapour compression refrigeration systems, the major operating cost is the energy input to the system in the form of mechanical work.

5. Any method of increasing coefficient of performance is advantageous so long as it does not does not involve too heavy an increase in other operating expenses, as well as initial plant cost & consequent maintenance.

6. The coefficient of performance of refrigeration system is the ratio of refrigerating effect to the compression work, hence coefficient of performance can be increased either by increasing the refrigerating effect or by increasing the compression work.

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1. The work done per kg of refrigerant is reduced in compound compression with intercooler.

2. It improves the volumetric efficiency for the given pressure ratio.

3. It reduces the leakage loss considerably.

4. It gives more uniform torque & hence smaller size flywheel is needed.

5. It gives effective lubrication because of lower temperature range.

6. It reduces the cost of compressor.

• Advantages of Compound VCRS :

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1. It is essential to superheat the vapour refrigerant leaving the evaporator so that no liquid may enter the compressor.

2. It possesses more wear, tear & noise due to moving parts of compressor.

• Disadvantages :

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• Vapour Absorption Refrigeration Systems (VARS)

1. Similar to vapour compression refrigeration systems however, required input to absorption systems is in the form of heat.

2. Absorptive refrigeration uses a source of heat to provide the energy needed to drive the cooling process.

3. Conventional absorption systems use liquids , called as wet absorption systems.

4. Invented by the French scientist Ferdinand Carré in 1858.The original design used water and sulfuric acid.

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5. In 1926, Albert Einstein and his former student Leó Szilárd proposed an alternative design known as Einstein refrigerator.

6. Popular alternative to regular compressor refrigerators where electricity is (i) unreliable, (ii)costly, or (iii)unavailable, or

where noise from the compressor is problematic, or where surplus heat is available (e.g., from turbine exhausts or industrial processes)

18Basic Principle Of Vapour Absorption System

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• Advantages of VARS:

1. Only moving part is pump, hence operation is smooth and essentially quiet.

2. Uses heat energy, mostly waste heat (e.g., factories, buildings, etc.) to change the phase of refrigerant in evaporator.

3. Load variations doesn’t affect the performance.

4. Can operate at reduced evaporator pressure and temperature by increasing the steam pressure to the generator.

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Water Spray Absorption Refrigeration

• Disadvantages of VARS:For the refrigeration and heat rejection temperatures, the COP of VCRS will be much higher than the COP of a VARS as a high grade mechanical energy is used in the former, while a low-grade thermal energy is used in the latter.

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• Industrial Applications:

1. Refrigeration is used to liquefy gases like oxygen, nitrogen, propane and methane.

2. Metal workers use refrigeration to temper steel and cutlery.

3. In oil refineries, chemical plants, and petrochemical plants, refrigeration is used to maintain certain processes at their required low temperatures (for example, in the alkylation of butenes and butane to produce a high octane gasoline component).

4. Refrigeration System for Dairy & Food Processing Industry.

(i)Ice Bank Tank Systems For Milk Chilling (ii)Cold Storage For Milk & Milk Products

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5. Refrigeration Systems For Pre-cooling & Cold Storages for Horticulture & Floriculture

6. Refrigeration System For Construction Industry

7. Refrigeration System for Poultry & fisheries

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Effect Of Cold Storage On Perishable Products

Photograph of a typical cold storage

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Ice is used to refrigerate and preserve food such as this Northern kingfish

Dunedin, the first commercially successful refrigerated ship.

Loading blocks of factory-made ice from a truck to an "ice depot" boat in the fishing harbor of Zhuhai, China

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Parameters

Air Refrigeration System(ARS)

Simple VCR System (SVCRS)

Compound VCR System (CVCRS)

VAR System(VARS)

Space Requirements

LARGE For given capacity MODERATE

LOWER than Simple VCRS

Favours LESS space requirement

Capacity Control

Easier, as lesser no. of Components

Automation slightly Difficult

Automation Difficult

Automated operation easy

COP LOW >then VARS<then CVCRS

HIGHER MODERATE

Energy Source

High Grade energy(mechanical work)

High Grade energy(mechanical work)

High Grade energy(mechanical work)

Low Grade Energy like Heat

• Comparative Study Of Different Refrigerating Systems :

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• Conclusion :

1. Vapour absorption system is better than the other two systems as_

(i)less moving parts, (ii)better performance, (iii)no leakage problems, (iv)better control over generator temperature,

and (v)less maintenance

Absorption Refrigeration System.

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1. C. P. Arora,” Refrigeration & Air-Conditioning(2000)“, Tata McGraw-Hill(New Delhi), 2nd Edition, pp.90.

2. NPTEL, “Lesson 9: Air cycle refrigeration systems “, Version 1 ME, IIT Kharagpur, pp.4.

3. NPTEL, “Lesson 10: Vapour Compression Refrigeration Systems “, Version 1 ME, IIT Kharagpur, pp. 8, 9.

4. Schmidt, R.R. and Notohardjono, B.D., “High-end server low-temperature cooling”, IBM Journal of Research and Development, Vol. 46, Nov.6, 2002.

5. Eric Granryd & Björn Palm, “Refrigerating engineering”, Royal Institute of Technology, Stockholm, 2005, see chap. 4-3.

6. Khurmi, Gupta, ” Refrigeration & Air-Conditioning(2010)“, S. Chand(New Delhi), 8th Edition(Multi-colour), pp.109, 242.

7. "Improved process for the artificial production of ice”, U.S. Patent Office, Patent 8080, 1851

8. “AIR REFRIGERATING MACHINE, 1881“, Machine-History.Com.9. Prof. U.S.P. Shet, Prof. T. Sundararajan and Prof. J.M .

Mallikarjuna(NPTEL), “Lesson 8: Refrigeration Cycles “, Version 1 ME, IIT Madras.

10. R. K. Rajput, “Refrigeration and Air-Conditioning”, 2006, 2nd Edition, pp.64, 65,79,80.

• References:

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